This invention is concerned with the purification of 2,2-bis(4-hydroxyphenyl) propane (hereinafter identified as "bisphenol-A," or "BPA"). More particularly, the invention involves continuous, counter-current treatment of a water/BPA crystal slurry, which is obtained from the aqueous crystallization of crude bisphenol-A, with an organic solvent, thereby simultaneously purifying the bisphenol-A crystals and extracting the bisphenol-A, phenol values and impurities present in the water. One embodiment of this invention can be performed in a continuous, multi-stage, counter-current extraction column.
Bisphenol-A is used in making polycarbonate resins by reaction of the latter with either phosgene or diphenyl carbonate, or for making epoxy resins, both resins being used extensively in commercial applications involving molding, casting, and sheet forming purposes. It is highly important that the monomeric bisphenol-A used to make such resins be as pure as possible in order to avoid adverse effects on the properties of the polymers.
Crude bisphenol-A is the product of commercial processes for preparing bisphenol-A. It is a mixture of bisphenol-A and impurities derived from the BPA synthesis reaction. An example of a reaction which produces crude BPA is the acid-catalyzed condensation of phenol and acetone where phenol and acetone react in the presence of an acidic material such as sulfuric acid, hydrochloric acid, cation exchange resin, etc.
The reaction is usually carried out in excess phenol (&gt;2 moles per mole acetone). The crude bisphenol-A is typically isolated in liquid form from the reaction mixture by a series of distillation steps.
The isolated crude bisphenol-A produced contains undesirable impurities such as the 2-(4-hydroxyphenol)-2-(2-hydroxyphenyl)propane (hereinafter identified as "o,p-isomer") having the formula: ##STR1## as well as other impurities including phenol itself used in making the bisphenol-A, a trishydroxyphenyl compound of the formula: ##STR2## (hereinafter identified as "BPX-1"), small amounts of other impurities such as the two compounds having the formulas: ##STR3## (hereinafter identified as "LD/CD") along with impurities of unknown structure with a yellowish color (herein identified as color bodies), etc.
Typically, the isolated crude bisphenol-A is crystallized in the presence of an organic solvent to free the impurities from the crude bisphenol-A. Crystallization in the presence of an organic solvent produces powdery fine needle-like crystals and permits occlusion by the organic solvent during their formation.
The fine needle-like crystals are difficult to handle, store and dry and, due to the organic solvent occlusion during the crystallization step, the dried product still contains some organic solvent.
A method which uses water as the crystallization medium for bisphenol-A is described in U.S. Pat. No. 3,326,986. According to this patent, the isolated crude bisphenol-A in molten form is mixed with water and the mixture is cooled to yield large, less needle-like crystals of bisphenol-A. Separation of these crystals from the mother liquor, followed by an organic solvent wash results in purification of bisphenol-A. Although the process described within the patent avoids occlusion by an organic solvent during the crystal formation step and yields large, less needle-like crystals, the purification may be limited due to limited washing of the crystals on solid/liquid separation equipment. Theoretically, it should be possible to provide a very effective washing of the crystals obtained from the water crystallization process by repeating the steps of crystal separation, reslurrying the crystals with the organic solvent and then crystal separation. However, such a process scheme is not viable since several complex operating steps and high costs are involved. An effective method of obtaining bisphenol of high purity from an aqueous crystallization process is described and claimed in copending application Ser. No. 443,344 filed Nov. 15, 1982, Mendiratta and Morgan). In this process aqueous crystallized crude bisphenol-A can be treated with an organic solvent while in the presence of water. The organic solvent removes the impurities and the water prevents the adhesion of the organic solvent onto the crystal's surface. The process described in copending application Ser. No. 443,344 filed Nov. 15, 1982 involves a batchwise contact of the water/BPA crystal slurry, produced from the aqueous crystallization step, with an organic solvent. This step is followed by agitation, settling and finally removal of the organic solvent to obtain purified bisphenol-A crystals.
The present invention demonstrates a new and improved process capable of purifying aqueous crystallized bisphenol-A with advantages over the process described in copending application Ser. No. 443,344 filed Nov. 15, 1982 which renders it more suitable for a bisphenol-A manufacturing operation.
One advantage of the present invention is that the extraction of bisphenol-A, phenol and impurities from the aqueous mother liquor and the organic solvent wash of a water/BPA crystal slurry, typically obtained form an aqueous crystallization step, can be accomplished simultaneously and continuously without interruption by intermittent steps, unlike a batchwise operation.
In addition, the present invention can purify bisphenol-A crystals and extract bisphenol-A and phenol from the aqueous mother liquor more efficiently and effectively, requiring less washing solvent than the process described in copending application Ser. No. 443,344 filed Nov. 15, 1982, since it is capable of contacting the water/bisphenol-A crystal slurry with organic solvent counter-currently in multiple stages. Also, the aqueous stream obtained after separation of bisphenol-A crystals can be recycled as such to the aqueous crystallization step, unlike the process disclosed in copending application Ser. No. 443,344 filed Nov. 15, 1982 where it is necessary to extract dissolved phenol, phenolic impurities and bisphenol-A from the recycled aqueous stream. Therefore the water can be recycled to an aqueous crystallization process once separated from the crystals without using additional organic solvent to extract phenol, phenolic impurities, and dissolved bisphenol-A before recycling, as required in the process discussed previously.